Exclude the specs on modern chef + fix a bad spec

This spec actually ran the code and it would fail if you had node
installed.
Signed-off-by: Tim Smith <tsmith@chef.io>

spec/spec_helper.rb

 require 'rspec'
 require 'chefspec'
 require 'chef/sugar'
+require 'chef/version'
 
 require_relative 'support/shared_examples'
 
@@ -22,4 +23,6 @@ RSpec.configure do |config|
 
   # ChefSpec configuration
   config.log_level = :fatal
+
+  config.filter_run_excluding pre_chef16_only: true if Gem::Requirement.new(">= 16.0.257").satisfied_by?(Gem::Version.new(Chef::VERSION))
 end

spec/unit/chef/sugar/shell_spec.rb

 require 'spec_helper'
 
 describe Chef::Sugar::Shell do
-  describe '#which' do
+  describe '#which', :pre_chef16_only do
     it 'returns the first executable matching the command' do
       allow(File).to receive(:executable?).and_return(false)
       allow(File).to receive(:executable?).with('/usr/bin/mongo').and_return(true)
@@ -46,7 +46,7 @@ describe Chef::Sugar::Shell do
     end
 
     it 'returns false if the given binary does not exist' do
-      allow(File).to receive(:executable?).and_return(false)
+      allow(described_class).to receive(:which).with('node').and_return(nil)
       expect(described_class.installed?('node')).to be false
     end
   end